The present application relates to reflow ovens and, in particular, cooling components used in reflow ovens.
Reflow ovens are used in the manufacture of printed circuit boards (xe2x80x9cPCBsxe2x80x9d) and other electronic devices, where electrical components are connected to the PCB and to each other by solder. In one step of a typical process for manufacturing a PCB, for example, a central processing unit (xe2x80x9cCPUxe2x80x9d) is connected to an interposer or substrate by placing solder paste on the interposer at the desired location for the CPU and placing the CPU onto the solder. The interposer is then placed on a conveyor belt that carries the substrate through the reflow oven. The reflow oven includes a heating section, where heat is generated to melt the solder, and a cooling section to cool and harden the solder, connecting the CPU to the PCB. An example of a reflow oven is the ATMOS 2000CR(trademark) Reflow Oven, available from Electrovert(copyright), Camdenton, Mo.
In the ATMOS 2000CR(trademark) Reflow Oven, cooling gas is provided to three cooling devices that emit the gas onto the PCB. The cooling devices in the ATMOS 2000CR are multi-piece elongated members including an internal longitudinal cavity and a longitudinal slit. Compressed gas is provided to the internal cavity and emitted through the slit in the form of a high velocity laminar flow onto the PCB""s as they are carried by the conveyor. FIG. 1 is a perspective view of a portion of a cooling device 10 used in the ATMOS 2000CR. Compressed gas is provided to the internal cavity of the cooling device 10 from a side of the device through a tube 11a and an adapter 11b. The gas is emitted through a longitudinal slit (not shown) on the PCBs. The cooling device 10 in the ATMOS 2000CR(trademark) Reflow Oven is an Air Knife(trademark) available from Exair Corporation, Cincinnati, Ohio.
Solder paste includes a deoxifying agent, referred to as flux, to remove oxides from the surfaces of the components to be soldered together. The flux typically becomes a vapor at a temperature below the temperature of the heating section and circulates throughout the oven. The vapor condenses on the cooling devices in the ATMOS 2000CR Reflow Oven, clogging the longitudinal slits through which the cooling gas exits the cooling devices. A heater 12 is therefore provided in contact with the cooling device 10 to heat the cooling device and melt the flux during periodic cleaning of the oven. The heater 12 is held against the cooling device 10 by two clips 14 that are bolted to each cooling device 10 through a hole 14a. FIG. 1 shows one such clip 14. The clip 14 has a 90xc2x0 bend 15 and a straight portion 16 that bears against the heater 12, pressing the heater 12 against the cooling device 10. A lip 18 extends perpendicular to the straight portion 16, adjacent to a side of the heater 12, as shown in the rear perspective view of FIG. 2 and in FIG. 3. The clip 14 is formed of a layer of spring steel having a thickness of about 0.889 mm. When connected within the ATMOS 2000CR(trademark) Reflow Oven, the assembly of the cooling device 10, the heater 12 and the clip 14, as shown in FIG. 1, is rotated 180xc2x0 about a longitudinal axis of the cooling device in FIG. 1 so that the heater is below the cooling device.
The inventor has found that over time the clip 14 may weaken, allowing the heater 12 to separate from the cooling device 10. The cooling device 10 may not, therefore, reach the optimal temperature for melting the flux, typically 200xc2x0 C. The cooling device 10 may then stay clogged, interfering with its ability to exhaust a sufficient quantity of cooling air to cool the PCB.
In the ATMOS 2000CR(trademark) Reflow Oven, thermocouplesare provided on each of the cooling devices. The duty cycles of the heaters 12 on all three cooling devices 10 are controlled by software based on the temperature sensed by the thermocouple on the middle of the three cooling devices 10. The inventor has also found that the heaters 12 may be turned off before the cooling device 10 closest to the exit reaches the optimal temperature for cleaning.